Field of the Disclosure
This disclosure relates to superconductors, and more specifically, to improving the critical current retention of a superconducting tape in a magnetic field.
Background of the Disclosure
Several materials and systems are being researched in order to solve looming problems with energy generation, transmission, conversion, storage, and use. Superconductors may provide a unique systemic solution for a broad spectrum of energy problems. More specifically, superconductors enable high efficiencies in generators, power transmission cables, motors, transformers and energy storage. Further, superconductors transcend applications beyond energy to medicine, particle physics, communications, and transportation. Superconducting tapes continue to be enabled by creating epitaxial, single-crystal-like thin films on polycrystalline substrates.
The current carrying capability of conventional superconductors rapidly diminishes in a magnetic field. This performance decay represents a potential technical hurdle for certain applications. An exemplary application may include wind turbine generators where the generator coil may be subjected to magnetic fields of a few Tesla. Additionally, since superconductivity in high-temperature superconductors (HTS) is localized within their Cu—O planes, HTS materials exhibit strong anisotropic behavior. This anisotropy is evident in critical current measurements when a magnetic field is aligned at different angles with respect to the film surface. It is observed that the critical current of a standard HTS tape drops rapidly as the field is moved away from the film surface and reaches a low value when the field is oriented approximately perpendicular to the tape. The reduction in critical current is the limiting value in coils constructed with these tapes. Thus, flux pinning or immobilizing the magnetic flux lines through the superconductor is one method of maintaining HTS tape performance. Flux pinning improvement strategies for practical superconductors have been researched over the last decade to improve performance in real world or “field” applications.
Conventionally, the most researched approach has been to introduce defects into the superconductor that are comparable in lateral dimensions with superconducting coherence length. In the second generation (2G) HTS tapes, representative defects may be oxygen vacancies, threading dislocations, twin planes, impurity atoms, irradiation-induced columnar defects, and nanostructured inclusions of various composition and structure. Recently, approaches for columnar defect formation based on chemically doping the superconducting film with BaZrO3 (BZO) or BaSnO3 (BSO) have been researched, where the BZO and BSO inclusions form nanosized column. These columns, about 5 nm in diameter, form by a self-assembly process during superconductor film growth and appear to improve the pinning strength.
In certain applications, such as power transmission cable, the magnetic field is aligned primarily parallel to the tape. It has been shown that in a magnetic field of 0.1 T aligned parallel to the tape surface, the critical current of a standard MOCVD-based 2G HTS tape decreases by about 20% to about 30%, in other words, only between about 70 to 80% of the zero-field critical current is carried in the HTS tape. Comparatively, a first-generation HTS tape based on (Bi, Pb)—Sr—Ca—Cu—O has been found to retain over 90% of its critical current in a field of 0.1 T applied parallel to the tape. Thus, the critical current retention of 2G HTS tapes in magnetic fields applied parallel to the tape represents a potential hurdle to industrial application.